1. Field of the Invention
The present invention refers to a starting circuit for switching power supplies (SMPS), to a switching power supply comprising a starting circuit and to a switching power supply integrated circuit.
2. Description of Related Art
Most of the control integrated circuits for switching power supplies require for their starting a current to charge a capacity of the self supply circuit of the integrated circuit. Such current comes from the starting circuit or also known as the bootstrap circuit (start-up), constituted in the simplest case by a resistance connected to the supply line.
When the voltage on the capacity achieves a preset voltage value called start-up voltage, the control circuit is supplied by the self supply circuit. The self supply circuit is generally constituted by an additional winding performed on the main transformer of the switching power supply to which a suitable rectification and filtration circuit is connected.
Since lately there are more and more severe requests for the reduction of the supply consumption, the simple above mentioned resistance is replaced by circuits active during the starting phase and inactive during the normal operation.
Lately there are also demands to have some monolithic devices for the switching power supplies that comprise both the control circuit and the starting circuit (active).
However, the technologies used for these integrated circuits, for instance that denominated RESURF (REduced SURface Field), have limitations as regards the maximum sustainable voltage between source/body of the transistor DMOS and the substrate. The control of the charge present in the epitassial layer, necessary for the component sustainable voltage, does not allow to guarantee punch-through voltages higher than about 20V.
The realization of thick epitassial layers allows to increase the punch-trough voltage, but it has as result the complication of the industrial process and a cost increase.
Therefore, a need exists to overcome the problems with the prior art as discussed above.